Here is a link to the pro audio discussion of the Audio Asylum forum. I'd ask a question to this forum.
Found a solution
Tascam MP GT1 portable guitar trainer that would blend your guitar with recorded music, and if necessary record it over so you can loop it.
Not sure if conventional guitar amps(or acoustic guitar amps) have monitoring capabilities of mixing line-level signal with the instrument. The one I have is the practice Fender 10W SS amp that doesn't have such features.